The present invention relates to an ink-jet recording head for ejecting ink droplets by displacing a piezoelectric element, in which a vibration plate constitutes a part of a pressure generating chamber communicating with a nozzle orifice that ejects ink droplets, and the piezoelectric element is provided through the vibration plate. Moreover, the present invention relates to an ink-jet recording apparatus.
As an ink-jet recording head for ejecting ink droplets from a nozzle orifice, in which a vibration plate constitutes a part of a pressure generating chamber communicating with a nozzle orifice that ejects ink droplets, and the vibration plate is deformed by the piezoelectric element to pressurize ink in the pressure generating chamber, the following two types have been put into practical use; one is an ink-jet recording head that uses a piezoelectric actuator of a longitudinal vibration mode, which stretches and contracts in an axial direction of the piezoelectric element, and the other one uses a piezoelectric actuator of a flexural vibration mode.
The ink-jet recording head of the former type has had an advantage that it can change a volume of the pressure generating chamber by allowing an end face of the piezoelectric element to abut on the vibration plate, thus making it possible to manufacture a head suitable for high-density printing. However, this type of ink-jet recording head has a problem of complicated manufacturing steps due to: a necessity of a troublesome step of cutting and dividing the piezoelectric element into a comb-tooth shape so as to coincide with an array pitch of the nozzle orifices; and a necessity of an operation of positioning and fixing the cut and divided piezoelectric elements onto the pressure generating chambers.
Meanwhile, the ink-jet recording head of the latter type has had an advantage that the piezoelectric element can be fixedly installed to the vibration plate through relatively simple steps of adhering a green sheet of a piezoelectric material to the vibration plate so as to match the pressure generating chamber in shape and of sintering the same. However, this type of ink-jet recording head has a problem of difficulty in arraying the pressure generating chambers in high density due to a necessity of a certain amount of area because of utilization of the flexural vibration.
In order to solve a disadvantage of the ink-jet recording head of the latter type, as disclosed in Japanese Patent Laid-Open No. Hei 5(1993)-286131, an ink-jet recording head has been proposed in which a piezoelectric material layer having an even thickness is formed over the entire surface of a vibration plate by a film growth technology, and this piezoelectric material layer is then cut and divided by a lithography method so that a shape of each piece of the layer can correspond to a shape of each pressure generating chamber, thus forming each piezoelectric element so as to be independent for each pressure generating chamber.
According to the ink-jet recording head as described above, advantages obtained are, not only that the operation of adhering the piezoelectric element to the vibration plate becomes unnecessary, and that the piezoelectric element can be fixedly installed to the vibration plate by a precise and simple method called the lithography method, but also that the piezoelectric element can be made thin to make a high-speed drive thereof possible.
Moreover, in this case, providing at least an upper electrode to each pressure generating chamber while leaving the piezoelectric material layer being provided on the entire surface of the vibration plate makes it possible to drive the piezoelectric element corresponding to each pressure generating chamber. However, it is desirable that a piezoelectric active portion having a piezoelectric layer and the upper electrode be formed so as not to be located outside the pressure generating chamber, since there are problems of a displacement amount per unit drive voltage and stress applied to the piezoelectric layer in a portion that straddles a portion facing towards the pressure generating chamber and outside thereof.
In this connection, a structure has been known in which an insulating layer covers the piezoelectric element corresponding to each pressure generating chamber, and a window (hereinafter, referred to as a contact hole) for forming a connection portion between each piezoelectric element and a lead electrode supplying a voltage to drive each piezoelectric element is provided in the insulating layer so as to correspond to each pressure generating chamber, thus forming the connection portion between each piezoelectric element and the lead electrode in the contact hole.
However, in the structure as described above in which the contact hole is provided for connecting the upper electrode and the lead electrode, there has been a problem that the entire film thickness of the portion provided with the contact hole becomes thick, thus lowering a displacement characteristic.
In order to solve the above-described problems, a structure has been proposed in which a piezoelectric non-active portion having a piezoelectric layer but not being substantially driven is provided in a region facing towards the pressure generating chamber in continuation with the piezoelectric active portion as a substantial drive portion of the piezoelectric element, thus forming the lead electrode without providing the contact hole.
However, in the structure as described above, the piezoelectric active portion becomes deformed when the piezoelectric element is driven by application of a voltage. And, there is a problem that damage such as a crack occurs in a boundary portion between the piezoelectric active portion and the piezoelectric non-active portion due to a drastic stress change generated therebetween.
Moreover, the above problem tends to occur particularly in the case where the piezoelectric material layer is formed by the film growth technology. This is because rigidity of the piezoelectric material layer is low in comparison with that of a piezoelectric material layer to which a bulk piezoelectric element is adhered since that the piezoelectric material layer formed by the film growth technology is very thin.
In consideration of circumstances as described above, the present invention has an object to provide an ink-jet recording head and an ink-jet recording apparatus in which damage to of the piezoelectric layer due to the drive of the piezoelectric element is prevented.
A first aspect of the present invention for solving the above-described problems is an ink-jet recording head, comprising: a pressure generating chamber communicating with a nozzle orifice; and a piezoelectric element having a lower electrode, a piezoelectric layer and an upper electrode, the piezoelectric element being provided in a region corresponding to the pressure generating chamber with a vibration plate interposed therebetween, wherein the piezoelectric element includes a piezoelectric active portion as a substantial drive portion and a piezoelectric non-active portion having the piezoelectric layer continuous from the piezoelectric active portion but not being substantially driven in a region facing to the pressure generating chamber, and a stress suppression layer for suppressing stress due to drive of the piezoelectric element is provided straddling a boundary between the piezoelectric active portion and the piezoelectric non-active portion.
In the first aspect, when the piezoelectric element is driven, the stress at the boundary between the piezoelectric active portion and the piezoelectric non-active portion of the piezoelectric element is suppressed, and damage to the piezoelectric layer is prevented.
A second aspect of the present invention is the ink-jet recording head according to the first aspect, wherein the piezoelectric layer has crystals subjected to a priority orientation.
In the second aspect, as a result of depositing the piezoelectric layer in a thin-film process, the crystals are subjected to the priority orientation.
A third aspect of the present invention is the ink-jet recording head according to the second aspect, wherein the piezoelectric layer has crystals shaped in a columnar shape.
In the third aspect, as a result of depositing the piezoelectric layer in the thin-film process, the crystals are shaped in the columnar shape.
A fourth aspect of the present invention is the ink-jet recording head according to any one of the first to third aspects, wherein the piezoelectric non-active portion is formed by removing the lower electrode.
In the fourth aspect, the piezoelectric non-active portion can be readily formed by removing the lower electrode.
A fifth aspect of the present invention is the ink-jet recording head according to any one of the first to fourth aspects, wherein a film thickness of the piezoelectric layer ranges from 0.5 to 3 xcexcm.
In the fifth aspect, the film thickness of the piezoelectric layer is made relatively thin, and thus the head can be miniaturized.
A sixth aspect of the present invention is the ink-jet recording head according to any one of the first to fifth aspects, wherein at least the piezoelectric layer constituting the piezoelectric element is independently formed in the region opposite with the pressure generating chamber.
In the sixth aspect, a displacement amount of the vibration plate due to the drive of the piezoelectric element is increased.
A seventh aspect of the present invention is the ink-jet recording head according to the sixth aspect, wherein a wiring electrode is extended from the upper electrode toward a region of a peripheral wall the pressure generating chamber.
In the seventh aspect, the upper electrode of the piezoelectric element and the external wiring can be connected relatively readily with the wiring electrode interposed therebetween.
An eighth aspect of the present invention is the ink-jet recording head according to the seventh aspect, wherein the wiring electrode also serves as the stress suppression layer.
In the eighth aspect, since the wiring electrode also serves as the stress suppression layer, a structure of the ink-jet recording head can be simplified, and a manufacturing cost thereof can be reduced.
A ninth aspect of the present invention is the ink-jet recording head according to any one of the first to eighth aspects, wherein the stress suppression layer includes an insulating layer made of an insulating material.
In the ninth aspect, the stress applied to the piezoelectric element is suppressed without short-circuiting the wiring of the piezoelectric element, and thus damage to the piezoelectric layer can be more securely prevented.
A tenth aspect of the present invention is the ink-jet recording head according to any one of the first to ninth aspects, wherein a width of an end portion of the stress suppressing layer on the piezoelectric active portion side is gradually reduced toward a tip thereof.
In the tenth aspect, since the stress applied to the piezoelectric element gradually changes in the vicinity of the boundary between the piezoelectric active portion and the piezoelectric non-active portion, damage to the piezoelectric layer due to the radical stress change at the boundary is prevented.
An eleventh aspect of the present invention is the ink-jet recording head according to any one of the first to tenth aspects, wherein the stress suppression layer is formed to have a width wider than a width of the pressure generating chamber in an outer region than the boundary between the piezoelectric active portion and the piezoelectric non-active portion, and the vibration plate in a region opposite with an edge portion of a longitudinal direction of the pressure generating chamber is covered with the stress suppression layer.
In the eleventh aspect, rigidity of the vibration plate is enhanced in the edge portion of the longitudinal direction of the pressure generating chamber, and thus damage to the vibration plate due to the drive of the piezoelectric element is prevented.
A twelfth aspect of the present invention is the ink-jet recording head according to any one of the first to eleventh aspects, wherein the pressure generating chamber is formed by subjecting a single crystal silicon substrate to anisotropic etching, and each layer of the piezoelectric element is formed of a thin film by a lithography method.
In the twelfth aspect, the pressure generating chamber can be formed relatively readily with high accuracy and high density.
A thirteenth aspect of the present invention is an ink-jet recording apparatus comprising the ink-jet recording head according to any one of the first to twelfth aspects.
In the thirteenth aspect, the ink-jet recording head can be realized in which durability and reliability of the head are improved.